Alteration of characteristic curve of zinc oxide electrophotographic materials



1961 J. G. JARVIS ETAL ,003, 70

ALTERATION QF CHARACTERISTIC CURVE OF ZINC- OXIDE ELECTROPHOTOGRAPHIC MATERIALS Filed April 17, 1957 3 Sheets-Sheet 1 Figzl DENSITY 0.8 LOG E FigzZ DENSITY 0.8 L06 E(RED) DENSITY L06 E L06 James G. J'izrV 'S Dale L.Slni ill RagenWT I8! INVENT RS 11'1" TORNEYS Oct. 10, 1961 J. G- JARVIS ETAL 3,003,870

ALTERATION OF CHARACTERISTIC CURVE OF ZINC OXIDE 'ELECTROPHOTOGRAPHIC MATERIALS Filed April 17, 1957 s Sheets-Sheet 2 ALISNEG J G J via 13173 1,. smith RayenWTgler INVENTORS ATTORNEY? Oct. 10, 1961 J. G. JARVIS ET AL 3,003,870

ALTERATION OF' CHARACTERISTIC CURVE UF ZINC OXIDE ELECTROPHOTOGRAPHIC MATERIALS Filed April 17, 1957 s Sheets-Sheet a %AREA COVERED BYSPECKS FASTER THAN BACKGROUND I00 80 60 4-0 O 7: AREA COVERED BY SPECKS SLOWER THAN BACKGROUND z AREA covmso BYDISCHARGED$CK$ 2% s i 1.0- I I x 20 -0 \/A 20.6 n 004 r so L. O 6 E CURVES SHOW EFFECT OF FULLY DISCHARG/NG THE INDICATED AREAS Figz? Jalnes G. Jarvis Dale .LSnuth INVENT ATTORNB YRS United States Patent 3,003,870 ALTERATION OF CHARACTERISTIC CURVE 0F ZRIHXSOXIDE ELECTROPHGTOGRAPHIC MATE- James G. Jarvis, Dale L. Smith, and Rayen W. Tyler,

Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Apr. 17, 1957, Ser. No. 653,316 Claims. (Ql. 96-1) This invention relates to electrophotography, and more particularly, to a means of altering the sensitivity characteristics of an electrophotographic material containing zinc oxide as the photoconductive ingredient.

It is known that zinc oxide can be dispersed in an insulating material, such as various synthetic resins of the polyvinyl type, waxy materials (e.g., paraffin, etc), etc, and this material coated on a conducting support, e.g., paper (sufliciently conducting at relative humidities above 30%). The resulting photographic element can then be given a blanket negative charge by placing the same under a corona discharge which serves to give a more or less uniform negative charge to the surface of the photoconductive layer. When the charged photoconductive layer is exposed imagewise to a light pattern, the negative charge leaks away roughly in proportion to the quantity of light falling upon that area of the surface of the photoconductive layer. The exposed material can then be treated with a developer containing a powder or toner on a granular carrier, e.g., iron filings, the powder being. attracted to those portions of the photoconductive layer which still retain an electrostatic charge. By selecting a low-melting solid. for one of the constituents of the powder, it is possible to treat the developed photoconductive material with heat and cause the powder to adhere permanently to the surface of the photoconductive layer. This method has been described as being somewhat similar to the process known as xerography, although it will be noted that the photoconductive layer itself provides the final print as distinguished from the xerographic process wherein the original photoconductive surface is used as a transfer plate to transfer the developed image to a separate receiving sheet. See RCA Review, vol. (1954), pages 469-484.

The zinc oxide customarily employed in the above process has sensitivity from about 325 to 410 mp, although its sensitivity can be extended considerably by special treatments with carbon dioxide or ammonia, or by sensitizing with a spectral sensitizing dye, such as Rose Bengal, iluorescein, cyanine dyes, etc. However, zinc oxide which may or may not have been treated to extend its light absorption range gives an H and D curve having rather steep gradation so that the exposure scale is generally no more than about 0.8 Log E unit (depending upon toner used as developer, etc.).

It is, therefore, an object of our invention to'provide an electrophotographic material having, an exposure scale considerably broader than the customarily employed electrophotographic materials comprising zinc oxide. Another object or our invention is to provide an electrophotographic material showing varying degrees of contrast, depending upon the spectral distribution of the exposing source. Still another object is to provide zinc oxide electrophotographic papers, the surface of which is sensitive to various regions of the spectrum.

Other objects will become apparent from a consideration of the following description. andv examples.

One of the present inventors has previously shown that the spectral sensitivity of a zinc oxide=coated paper is dependent primarily upon. the spectral characteristics ofa. thin layer at. the surface ot the photoconductive layer.

See Jarvis U.S. application Serial No. 630,480, filed Do.

cernber 26, 1956. In the earlier-filed application, the

surface layer is continuous, although in the instant in-.

The first method comprises treating the surface of a' zinc oxide photoconductive layer as described in greater detail hereinafter with a pattern of specks of a zinc oxide dispersion having a sensitivity difierent from that of the uniformly sensitive lower layer. This difierence in sensitivity may be a dilferent level of sensitivity, to the same region of the spectrum as the lower layer, or it may be a sensitivity in a different region of the spec trum. For example, the lower continuouslayer may be green sensitive, while the pattern of specks may have its maximum sensitivity in the red region of the spectrum. On the other hand, the lower layer may have its sensitivity in the green region of the spectrum, While the pattern of specks. may have a different level of green sensitivity. Another means of providing the novel com positions of our invention is to sensitize by means of a non-wandering dye, a zinc oxide dispersion and this sensitized dispersion intimately with another zinc oxide dispersion which may or may not have been Sensitized (its sensitivity being different from that of the our invention is dependent upon three variables, which are:

(1) The difierence in sensitivity between the background and the specks.

(2) The size of the specks, and v (3) The percentage of area covered by the specks.

The novel features of our invention are set forth in more detail below, and with particularity, in the following claims. The invention itself, both as to its organization and methods of operation, will best be understood from the following description of embodiments when read in connection with theaccompanying drawing in which:

FIGURE 1 is a diagrammatic sketch of the characteristic curve of a typical electrophotographic material prepared according to prior art methods. 7 g

FIGURE 2 shows the characteristic curve of ail electrophotographic material obtained according to our invention upon exposure of this material to red illumination. 7

FIGURE 3 illustrates the characteristic curve ofan electrophotographic material obtainable according to our invention and shows the elfect ofexposure of this material to red or green illumination.

FIGURE 4 illustrates the characteristic curve of the same material shown in FIGURE 3', the ratio of red green illumination being adjusted,- as described more fully, below. 7 7

FIGURE 5 illustrates the sensitometr'ic' properties of the material obtained according to Example 1 below, as compared with uniformly sensitized electropho'tographic material.

FIGURE 6 illustrates the relationship between speck coverage and integrated density, as described in more detail below, and

FIGURE 7 illustrates a family ofcliaract'eristic curves wherein the speck coverage is varied as described in more detail below.

Patented Oct. 10, 1961 Where the background and specks have their sensitivity lying in the same region of the spectrum, both have generally been sensitized by means of a spectral sensitizing dye as described in more detail below. In general, the background and specks will have a sensitivity differing from one another on the order of about 6 to 8 times as a maximum to about 1% times as a minimum. That is, the sensitivities of the two areas will differ from one another on the order of not more than about 0.8 Log E unit and not less than about 0.1 Log E unit. This means that the shoulder of the characteristic curve of one area will correspond approximately to the toe of the characteristic curve of the other material for the maximum case. Of course, it is possible to select sensitivities so that the two characteristic curves are not continuous but have a plateau between them. In general, such a material would have only special applications and not be as widely useful as the materials having a continuous exposure scale. Of course, where the background and the specks have their sensitivities in a different region of the spectrum, the above conditions need not apply, since these areas are not sensitive to the same spectral areas, and it is possible to use an exposing source having a mixture of radiations to which the areas are sensitive. By varying the proportions of light to which each region is sensitive, it is possible to obtain a variety of curve shapes and interesting effects.

In general, we have found that the percentage of area covered by the specks should vary between about 2 and 98 percent, based on the total area of the photoconductive surface. The specks can vary in number from about 10 to 300 per square inch. This corresponds roughly to a halftone screen ruling from about 10 lines per inch, such as might be used for billboard posters, to about between 200 and 300 lines per inch for high quality reproduction. While the foregoing applies primarily to coatings prepared by applying a pattern of specks to a continuous photoconductive coating, so that the material can be said to be composed of a background area and an area of specks, it is to be understood that similar characteristics are necessary where the photoconductive surface is prepared from a mixture of two or more zinc oxide dispersions having different sensitivities.

In the copending applications, Kendall and Stewart 630,463, filed December 26, 1956, and Stewart Serial No. 630,462, filed December 26, 1956, there are described methods of spectrally sensitizing zinc oxide dispersions. Other dyes useful for this purpose are described in the RCA Review article mentioned above. Typical dyes for conferring sensitivity in the green region of the spectrum are:

A. Merocyanine dyes (1) 3-B-carboxyethyl 2 (3,3 dicyanoallylidene)benzothiazoline (2) 3-carboxymethy1- 2 (3,3 dicyanoallylidene)benzothiazoline (3) 3-carboxymethyl-5-[(3-methyl 2(3H) thiazolinylidene)isopropylidenehhodanine (4) 3-ethyl-5-[(3-carboxymethyl 2(3H) benzoxazolylidene)ethylidene]rhodanine (5) 5- 3-ethyl 2 3H) benzoxazolylidene) ethylidene] 3-p-sulfoethyl-2-thio-2A 3 ,5) -oxazoledione (6) 3-carboxymethyl-5-[(3-ethyl 2(3H) benzothiazolylidene ethylidene] rho danine (7) 4-[(3-ethyl 2(3H) benzothiazolylidene)isopropylidene] -3-methyl-1 (p-sulfophenyl -5-pyrazolone (8) 3-(2,5- disulfophenyl) 5 [(3-ethy1- 2(3H) benzothiazolylidene) ethylidene] rhodanine B. Cyanine dyes (9) 3,3-diethyl-4,5,4,5'-dibenzothiacyanine chloride 10) 3-B-carboxyethyl-1'-ethyl 6' methoxy 5 phenylthia-2'-cyanine iodide (11) 3,3'-diethyl-4,5,4',5'-dibenzoxacyanine iodide (12) 3,3'-diethylthiazolinocarbocyanine iodide (13) 3,3'-diethyloxacarbocyanine iodide (14) 3,3'-diethyl-9-methyloxaselenacarbocyanine 1od1de Typical dyes for conferring sensitivity in the red region of the spectrum are:

A. Dicarbocyanine dyes (15) 3,3-di-B-hydroxyethylthiadicarbocyanine bromide (16) Anhydro 3,3'-di-13-carboxyethylthiadicarbocyanine hydroxide (17) 3,3'-diethyloxathiadicarbocyanine 1od1de D (18) 3,3-diethyl-4,5,4',5-dibenzothiadicarbocyanme 1odide (19) 3'-carboxymethy1-3-ethyloxathiadicarbocyanine iodide (20) 3-carboxymethyl-3-ethyloxathiadicarbocyanine iodide (21) 3,3-di(carboxymethyl)oxathiadicarbocyaniue bromide B. Carbocyanine dyes (22) 3,3'-diethyl 9 methyl-4,5,4',5-dibenzothiacarbocyanine chloride (23) Anhydro-3,3 di ,8 carboxyethyl-5,5-dichloro-9- ethylthiacarbocyaniue hydroxide (124) Anhydro 3 ,B carboxyethyl-S,5'-dichloro-9-ethyl- 3- 3-sulfoethylthiacarbocyanine hydroxide (25) 9-ethyl-3,3-di-,8-hydroxyethy1thiacarbocyanine 1odide C. Complex merocyanine dyes (26) 2-(3-carboxymethyl 4 oxo 2 thiono5-thiazolidylidene)-3-ethyl 5 [(3 ethyl-2(3H)-benzoxazolylideue) ethylidene] -4-thiazolidone (27) 3-ethyl 5 [(l-ethyl 4(1H) quinolylidene) ethylidene1-2-(3-ethyl 4 oxo 2 thiono 4 thiazolidylidene)-4-thiazolidone (28) 2-(3 carbethoxymethyl 4 oxo 2 thiono-S-th azolidylidene) 3 ethyl-5-[(3-ethyl 2(3H) benzothiazolylidene) -a-ethylethylidene] -4-thiazolidone (29) 5-[(l-ethyl 2(1H) p naphthothiazolylideneyaphenylethylidene]-3-p-methoxyethyl 2 (3 p methoxyethyl 4 oxo 2 thiono-S-thiazolidylidene)-4th1 azolidone (30) 5-[(1-ethyl 2(1H) 18 naphthothiazolylidene)-aethylethy1idene1-3 ,6 methoxyethyl 2 (3 [3 methoxyethyl- 4 oxo 2 thiono 5 thiazolidylidene-4-tluazolidone Of course, our invention is not dependent upon any particular group of spectral sensitizing dyes, although it Wlll be understood that some dyes are more useful than others in providing higher levels of sensitivity.

The unique effects of the photoconductive layers of our invention can be illustrated by reference to the accompanying stylized or schematic drawings. In FIGURE 1 is shown the characteristic curve of a typical zinc oxide coating which has been made green sensitive. FIGURE 1 shows that the maximum density obtainable with such material is 1.2 when developed with a typical toner, while its exposure scale covers only 0.8 Log E unit. The photoconductive material in FIGURE 1 consists of a continuous layer having more or less uniform sensitivity. Each part of the surface area of such a sheet is then covered with a pattern of specks having red sensitivity and occupying approximately 20.3 percent of the surface area. A material is obtained having the characteristic curve shown in FIGURE 2, if the material is exposed to red illumination. In the region A on the curve shown in FIGURE 2, the red sensitive specks have become completely discharged, while the green sensitive areas of the background have lost no charge and can still be developed to a micro maximum density of 1.2. Since the red sensitive specks occupy 20.3 percent of the surface area, the sample will have an integrated reflectance of about 25 percent, corresponding to a macro or integrated density of about 0.6. Of course, by controlling the percentage of area covered y he specks, the density of region A can be placed at any desired level.

If the sample shown in FIGURE 2 is now given a weak green exposure, a characteristic curve as shown in FIG- URE 3 will be obtained.

By controlling the ratio of red-to-green exposure of the material shown in FIGURE 2, a large variety of curve shapes can be obtained. By adjusting the red-togreen ratio, it is possible to obtain a curve as shown in FIGURE 4, for which the red-green ratio causes the plateau at B of FIGURE 3 to vanish. While the drawings in FIGURES l to 4 are diagrammatical, they are typical for the process, and it can be seen that the exposure scale of the paper has been doubled, while the gamma or contrast has been halved.

Instead of making two separate exposures as mentioned above, the sample can be illuminated by a mixture of red and green light. This can easily be obtained from an incandescent light source filtered by magenta and cyan color compensating filters. Inasmuch as the red and green sensitive areas shown in the characteristic curves of FIGURES 1 to 4 are also sensitive to ultraviolet radiation, and about equally so, the ultraviolet component should be filtered out if maximum exposure scale is to be obtained. By exposing the material illustrated in FIGURES 1 to 4 by ultraviolet radiation alone, a curve shape similar to that of FIGURE 1 can be obtained. By selecting appropriate mixtures of ultraviolet, green and red in suitable proportions, any curve between FIGURES 1 and 4 can be obtained. By using more than two sensitive components in the system, it is possible to extend the exposure scale to an extent even greater than that shown in FIGURE 4.

The following examples will serve to illustrate the manner of preparing coatings useful in our invention, as well as means for exposing such coatings to obtain maximum exposure scales.

EXAMPLE 1 100 parts of powdered zinc oxide were mixed with 20 parts of a high styrene content styrene-butadiene resin sold commercially as Pliolite S-7 (Goodyear Rubber & Tire Co.) and sufficient toluene added to the viscous dispersion to give 50 percent in total solids. Rose Bengal (0.02 part) was added to the viscous solution which was then coated on a photographic paper base (baryta coated). The toluene was then evaporated from the coating to give a photoconductive layer comprising zinc oxide, binder and sensitizing dye. It was green sensitive.

A second dispersion of zinc oxide was prepared exactly as described above, except that 0.01 part of 3,3'-di(fihydroxyethyl)thiadicarbocyanine bromide was used as the sensitizing dye in place of the Rose Bengal. The dispersion was then diluted with toluene to 22.5 percent in total solids and this. dispersion sprayed by a few rapid passes. of a finely-adjusted, spray gun onto the surface of the dried photoconductive coating. The amount of dispersion laid. down was. estimated by weighing. The speck size was found to be approximately 0.1 to 0.2 mm., while the amount of specks added was about 0.10 g./square foot, corresponding to an area coverage of 20.3 percent. The solvent. was then evaporated. from the speck areas of the coating.

The eltect of exposing the above photoconductive paper is illustrated in FIGURE of the accompanying drawings. Curve 1 of FIGURE 5 illustrates the characteristic curve obtainedfor this material using only ultraviolet radiation for the exposure. For this purpose, ex.- posure was made through a Wratten No. 18A filter, i.e., a filter transmitting radiation only between about 285 and 405 m Curve 3 of FIGURE 5 illustrates the characteristic curve obtained for this material when exposure is made to 3000- K. tungsten illumination filtered by a Kodak, Wratten; filter No. 8 (which transmits radiation only beyond about 460 mp) and two pieces of CC40M (Kodak Wratten Filters, 18th edition, published by Eastman Kodak Company, 1951,'page 71) to balance the red-green exposure. trates the characteristic curve obtained for the same material without the surface specks, i.e., green-sensitive only. It can be seen by comparing curve 2 with curve 3 that an exposure scale of about 1.8 Log E was reached for the material having surface specks as compared to about 0.9 Log E for the same material without the surface specks. It will be further noted that in curve 3 the red-green exposure was adjusted to eliminate the plateau region defined by B of FIGURE 3.

EXAMPLE 2 An electrophotographic paper having performance characteristics similar to the paper described in Example 1 above was prepared by spraying the red sensitive specks on a green sensitive support layer which was prepared by the following method:

parts of zinc oxide were mixed with 20 parts of a styrene-butadiene resin of the type illustrated in Example 1 and sufi'icient toluene added to obtain a solid concentration of 10 percent. Rose Bengal (0.02 percent based on the amount of zinc oxide present) was then added and the mixture was sprayed onto a baryta-coated photographic paper base which had been previously coated with the same zinc oxide dispersion containing no Rose Bengal. The coverage of the sensitized, sprayedlayer was 0.5 g. of the dried coating per square foot, whereas the coverage of the unsensitized layer was about 1 to 2 g. per square foot. Such a green sensitive coat- .ing is shown in the copending application of J. G. Jarvis,

erial No. 630,480, filed December 26, 1956.

EXAMPLE 3 100 parts of powdered zinc oxide were mixed with 20 parts of a high styrene content styrene-butadiene resin of the type used in Example 1 above, and suflicient toluene was added to produce a dope containing 50 percent in total solids. This dope was then coated on an ordinary photographic paper support containing a conventional baryta layer. The toluene was then evaporated fromthe coating.

An identical dope of zinc oxide was prepared and 0.10 parts of Rose Bengal added. The sensitized dope was then diluted to 20 percent total solids with toluene, and the diluted dope applied with a spray gun to the unsensitized coating so that the coverage of green sensitive specks was about 25 percent of the surface area. The treated coating was then dried thoroughly.

The electrophotographic paper prepared in this man.- ner was found to have an exposure scale of about 1.3 Log E units compared with a uniformly sensitive, unsensitized layer having an exposure scale of about 0.7 Log E unit. The unsensitized layer had normal sensitivity to ultraviolet iilumination, while the pattern of specks was sensitive both to ultraviolet and green illumination. By varying the ratio of ultraviolet to green illumination, it was possible to produce a large variety of curve shapes. Unfiltered tungsten illumination of 3000 K. was found to provide a useful ultraviolet-green ratio giving the desired exposure scale of 1.3 Log E units. Variations in the ultraviolet-green ratio can be made using a Kodak Wratten filter No. 8 and a Kodak Wratten filter such as No. 18A, which transmits radiation only between about 285 and 405 m the filters being superposed on separate tungsten light sources in order to control. mixing.

EXAMPLE 4 A layer of green sensitive zinc oxide was prepared exactly as described in Example 1 above, this coating con taining about 0.02 part of Rose Bengal. This composition was then coated on an ordinary photographic paper support containing the conventional ba'ry-ta layer. A second. green sensitive zinc oxide dispersion was prepared as follows:

100 parts of powdered zinc oxide were mixed with 20 Curve 2 of FIGURE 5 illus parts by weight of a high styrene content styrene-butadiene resin of the type illustrated in Example 1 and this mixture dissolved in toluene to give 50 percent total solids. Rose Bengal (.08) was then added to the zinc oxide composition and the mixture diluted with toluene to give about 22.5 percent total solids. This green sensitive zinc oxide dispersion, having a higher level of green sensitivity than the previously coated green sensitive zinc oxide layer was then sprayed with a spray gun onto the previously coated material to give a speck coverage of about 20 percent.

By varying the degree to which the ultraviolet component has been excluded, it is possible to vary the exposure scale between 0.8 and 1.6 Log E units, using 3000 K. tungsten illumination for this particular electrophotographic material, i.e., the material of Example 4.

Another type of coating, similar to that of Example 4, comprises an ultraviolet sensitive zinc oxide layer coated with specks of a dispersion having a dilferent ultraviolet sensitivity. The difference in ultraviolet sensitivity can be obtained by dyeing the binder in either the support layer or the specks with an ultraviolet absorbing dye of the type illustrated in Sawdey U.S. Patent 2,739,888, issued March 27, 1956. Of course, either region could also contain a spectral sensitizing dye, preselected to give different sensitivity in the background and speck portions of the coatings but the sensitivity in the specks and the background should have the same relative values (i.e., ratio of ultraviolet to sensitized) throughout the ultraviolet and visible spectrum so that such a coating is invariant with respect to the spectral quality of the illuminant and can be manufactured in specified contrast grades.

EXAMPLE An unsensitized zinc oxide dispersion was prepared exactly as described in Example 3 above and 5 parts of this unsensitized dispersion was mixed with 1 part of a green sensitive zinc oxide dispersion prepared according to Example 4 above, this dispersion containing 0.01 part of Rose Bengal as the green sensitizer. The mixed dispersions were then coated out onto an ordinary photographic paper support having a baryta layer. After removal of the toluene by heating, the exposure scale of the resulting paper was extended, compared with a similar material which had been uniformly sensitized.

Improved results can be obtained by employing a spectral sensitizing dye having non-wandering properties, such dyes generally containing a long-chain alkyl group. Typical of such non-wandering dyes which can be usefully employed is 5-[ (3-ethy1-2( 1H) -a-naphthothiazolylidene) ethylidene] -3 -n-hep tyll-phenyl-Z-thiohyd antoin.

Instead of applying the pattern of specks by means of a spray gun using a dilute zinc oxide dispersion, it is possible to sensitize selected areas of the original zinc oxide coating by direct application of sensitizing dye, such as by letterpress or lithography, silk screen, gravure, etc. Under such conditions, care should be taken to avoid diffusion of the dye away from the area to which it is originally applied.

The relationship between the speck coverage and integrated density for the discharged sensitivity specks on a fully charged background can be explained in the following way. For a typical developing powder the maximum density of the process is 1.20, corresponding to a reflectance of 6.3 percent, while the minimum density (the paper being substantially completely discharged) is 0.10 corresponding to a reflectance of 79.3 percent. It can be safely assumed that the specks become completely dis charged (corresponding to a micro minimum density of 0.10) while the remaining areas are still fully charged (corresponding to a micro maximum density of 1.20). If the surface area (uniformly sensitized) is then covered to the extent indicated above, i.e., between 2 and 98 percent, with sensitivity specks, the corresponding integrated densitiescan be tabulated as follows:

Percent Aroa C VEI'B Integrated density when specks by Specks: are completely discharged The above figure are plotted in FIGURE 6 of the accompanying drawings. A sample calculation for these data is as follows:

If area covered by specks=5 percent Then remaining area=95 percent Reflectance of specks=5/10O x 79.3 =3 .96 Reflectance of remaining area=95/100 6.3=5.98

Total or integrated reflectance =9.94

Corresponding Integrated Density =1.002

In FIGURE 7, the efiect of coverage from 2 to 98 percent on the characteristic curve of the original material is shown diagrammatically. In curve A of FIGURE 7 only two percent of the area is covered by specks having a sensitivity different from the background, giving a long shoulder to the characteristic curve. Curve B shows a corresponding material but the coverage of the specks is 70 percent, giving a curve having a long toe. In these two cases (curve A and curve B), the specks are exposed in such a way that they appear to be faster than the background. Of course, by choosing a material for specks that could be made to appear slower than the background, the same curves could be obtained for about 98 percent and 30 percent coverages, respectively.

It can be seen by reference to FIGURE 7, that useful results are obtained according to our invention when the percentage area covered by the specks is as low as 2 percent or where coverage is as high as 98 percent.

As indicated above, it is frequently desirable to employ sensitizing dyes in our invention which are characterized by their non-wandering properties. See Example 5 above. Such dyes generally contain an alkyl group having at least 7 carbon atoms and a group of such dyes have been previously described in U.S. Patent 2,282,116. Other examples of such dyes can be found in additional patents, domestic and foreign.

The term polyvinyl resin" used in the accompanying claims means a polymer obtained by polymerizing a composition containing a vinyl monomer, alone or in admixture with another polymerizable material, such as butadiene. Such vinyl monomers contain a polymerizable CH =CH group.

Certain of the resinous insulating materials customarily employed as a vehicle for the zinc oxide in elcctrophotographic processes have the disadvantageof sulfering from a rather rapid dark decay rate (after the charged zinc oxide layer has been exposed). It has been found that this difiiculty of high dark decay rate can be materially alleviated by adding a small amount (0.1 to 1.0 percent, or more) of a metallic drier to the resinous binder composition. Typical driers include lanthanum naphthenate, cerium naphthenate, zirconium naphthenate, cobalt naphthenate, lead naphthenate, lanthanum 2-ethylhexoate, cerium 2-ethylhexoate, etc., these metallic driers being used alone or in admixture with one another. These metallic driers are particularly useful in treating resinous binders, such as drying oil-modified styrene alkyd resins, silicone alkyd resins, etc.

What we claim seam invention and desire secured by letters" Patent" of the United States is: g V

1 An electrophotographio element comprising a con tinuous" coating on a conductive snpport of particulate photoconductive zinc oxide (1') having a given sensitivity, dispersed in an insulating binder-material, and coated over said continuous coating a discontinuous coating comprising small spots of substantially,- uniformly-distributed insulating binder-material containing particulate photoeonductive zinc oxide (II) having a ditferent givenflsensitivity, said particulate photoconductive zinc oxide (1) and said particulate photoconductive zinc oxide (II) having sensitivity in the same spectral region; but one of said particulate photoconductive zinc oxides having a sensitivity of from. 1% to 8 times that of the other of said particulate photoconductive zinc oxides, from 2 to 98% of the surface of said continuous coating being coated With said small spots of substantially,- uniformly-distributed insulating binder-material containing particnlate pliotoconductive zinc oxide (II), the number of said small spots being between and 300 per square inch, the said insulating binder-materials of said coatings being of such nature that said coatings have a low decay rate in theydark when said coatings have received an letfdstatic charge.

, 2. An electrophotographic element comprising a continuous coating on a conductive support of part u'late photoconductive zinc oxide (I) having a given sensitivity, dispersed in an insulating binder-material, and coated over said continuous coating a discontinuous coating comprising small spots of substantially, uniformly-distributed insulating bindernnaterial containing particulate photoconductive zinc oxide (II) having a different given sensitivity, said particulate photoconductive zinc oxide (I) and said particulate photoconductive zinc oxide (II) having sensitivity in dififerent spectral regions and one of said particulate photoconductive zinc oxides having a sensitivity lying beyond about 410 mu, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly-distributed insulating binder-material containing particulate photoconductive zinc oxide (II), the number of said small spots being between 10 and $00 per square inch, the said insulating binder-materials of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

3. An electrophotographic element comprising a continuous coating on a conductive support of particulate photoconductive zinc oxide (I) having a given sensitivity, dispersed in an insulating polyvinyl resin, and coated over said continuous coating a discontinuous coating comp1ising small spots of substantially, uniformly-distributed insulating polyvinyl resin containing particulate photoconductive zinc oxide (II) having a different given sensitivity, said particulate photoconductive zinc oxide (I) and said particulate photoconductive zinc oxide (II) having sensitivity in the same spectral region, but one of said particulate photoconductive zinc oxides having a sensitivity of from 1% to 8 times that of the other of said particulate photoconductive zinc oxides, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly-distributed insulating polyvinyl resin containing particulate photoconductive zinc oxide (II), the number of said small spots being between 10 and 300 per square inch, the said insulating polyvinyl resins of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

4. An electrophotographic element as defined in claim 3 wherein the conductive support is paper.

5. An electrophotographic element comprising a continuous coating on a conductive support of particulate photoconductive zinc oxide (I) having a given sensitivity, dispersed in an insulating polyvinyl resin, and

. f t) a coated over said e'ntifiueus dusting discontinuous coat; ing comprising small spots of substantially, ufiitonnry distributed insulating polyvinyl resin containing" particulate photoconductive zinc oxide (II having a different given sensitivity, said particulate photoconductive zinc oxide (I) and said particulate photoc'onductive zinc oxideII-I) having sensitivity in different spectral regionsand one of said particulate photoconductive' zinc oxides having" a sensitivity lying beyond about 410 mu, from 2. to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly distributed insulating polyvinyl resin containing particulate photoconductive Zinc oxide (II), the number of said small spots being between 10 and 300 per square men, the said insulating polyvinyl resins of said coatings beirig of SllCh nature that saidcoatings have a low decay rate irithe dark when said coatings have received an electrostatic charge.

6. An elect-rophotog'raphic element comprising a eontinuous coating on a conductive support of particulate photoconductive zinc oxide (I) having a. given sensitivity, dispersed in an insulating polyvinyl resin, and coated over said continuous coating a disont'inuous coating campus ing small spots of substantially, uniformly-distributed insulating polyvinyl resin containing particulate photoconductive zinc oxide (II) having a diiferent given sensitivity, one of said particulate z'inc oxides having sensitivity only in the red region of the visible spectrum and the other of said particulate zinc oxides having sensitivity only in the green region of the visible spectrum, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly distributed insulating polyvinyl resin containing particulate photoconductive zinc oxide (II), the number of said small spots being between 10 and 300 per square inch, the said insulating polyvinyl resins of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

7. An electrophotographic element comprising a continuous coating on a conductive support of particulate photoconductive zinc oxide (I) having a given sensitivity, dispersed in an insulating polyvinyl resin, and coated over said continuous coating a discontinuous coating comprising small spots of substantially, uniformly distributed insulating polyvinyl resin containing particulate photoconductive zinc oxide (II) having a different given sensitivity, one of said particulate photoconductive zinc oxides having sensitivity only beyond the blue region in the visible spectrum and the other of said particulate photoconductive zinc oxides being unsensitized, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly distributed insula ing polyvinyl resin containing particulate pliotoconductive zinc oxide (II), the number of said small spots being between 10 and 300 per square inch, the said insulating polyvinyl resins of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

8. An electrophotographic element, as defined in claim 7, wherein the sensitized photoconductive zinc oxide particles are sensitized with a merocyanine dye having an alkyl group containing at least 7 carbon atoms attached to the carbonylic oxygen-containing nucleus of said merocyanine dye.

9. An electrophotographic element comprising a continuous coating on a paper support of particulate photoconductive zinc oxide (1) having a given sensitivity, dispersed in an insulating binder-material, and coated over said continuous coating a discontinuous coating compris ing small spots of substantially, uniformly-distributed insulating binder-material containing particulate photoconductive zinc oxide (II) having a different given sensitivity, said particulate photoconductive zinc oxide (I) and said particulate photoconductive zinc oxide (II) having sensitivity in the same spectral region, but one of said particulate photoconductive zinc oxides having a sensitivity of from 1% to 8 times that of the other of said particulate photoconductive zinc oxides, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly-distributed insulating binder-material containing particulate photoconductive zinc oxide (11), the number of said small spots being between 10 and 300 per square inch, the said insulating binder-materials of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

10. An electrophotographic element comprising a continuous coating on a paper support of particulate photoconductive zinc oxide (I) having a given sensitivity, dispersed in an insulating binder-material, and coated over said continuous coating a discontinuous coating comprising small spots of substantially, uniformly-distributed insulating binder-material containing particulate photoconductive zinc oxide (II) having a different given sensitivity, said particulate photoconductive zinc oxide (I) and said particulate photoconductive zinc oxide (11) having sensitivity in different spectral regions and one of said particulate photoconductive zinc oxides having a sensitivity lying beyond about 410 me, from 2 to 98% of the surface of said continuous coating being coated with said small spots of substantially, uniformly-distributed insulating binder-material containing particulate photoconductive 12 zinc oxide (11), the number of said small spots being between 10 and 300 per square inch, the said insulating binder-materials of said coatings being of such nature that said coatings have a low decay rate in the dark when said coatings have received an electrostatic charge.

References Cited in the file of this patent UNITED STATES PATENTS 2,318,597 Davey at al. May 11, 1943 2,331,444 Wainer Oct. 12, 1943 2,358,060 Davey Sept. 12, 1944 2,703,282 Carroll Mar. 1, 1955 2,727,808 Thornsen Dec. 20, 1955 2,737,766 Hill Feb. 21, 1956 2,862,815 Sugarman et al. Dec. 2, 1958 FOREIGN PATENTS 314,838 Great Britain July 3, 1929 201,301 Australia Mar. 19, 1956 201,416 Australia Apr. 13, 1956 OTHER REFERENCES Young et al.: R.C.A. Review, December 1954, pp. 469-84.

Nelson: J our. of Optical Society of America, 46 (No. 1), 13-16 (1956).

Severance: Manual of Foreign Patents, pp. 12-17. The Patent Oflice Society, Washington, 1953. 

1. AN ELECTROPHOTOGRAPHIC ELEMENT COMPRISING A CONTINUOUS COATING ON A CONDUCTIVE SUPPORT OF PARTICULATE PHOTOCONDUCTIVE ZINC OXIDE (1) HAVING A GIVEN SENSITIVITY, DISPERSED IN AN INSULATING BINDER-MATERIAL, AND COATED OVER SAID CONTINUOUS COATING A DISCONTINUOUS COATING COMPRISING SMALL SPOTS OF SUBSTANTIALLY, UNIFORMLY-DISTRIBUTED INSULATING BINDER-MATERIAL CONTAINING PARTICULATE PHOTOCONDUCTIVE ZINC OXIDE (II) HAVING A DIFFERENT GIVEN SENSITIVITY, SAID PARTICULATE PHOTOCONDUCTIVE ZINC OXIDE (I) AND SAID PARTICULATE PHOTOCONDUCTIVE ZINC OXIDE (II) HAVING SENSITIVITY IN THE SAME SPECTRAL REGION, BUT ONE OF SAID PARTICULATE PHOTOCONDUCTIVE ZINC OXIDES HAVING A SENSITIVITY OF FROM 11/4 TO 8 TIMES THAT OF THE OTHER OF SAID PARTICULATE PHOTOCONDUCTIVE ZINC OXIDES, FROM 2 98% OF THE SURFACE OF SAID CONTINUOUS COATING BEING COATED WITH SAID SMALL SPOTS OF SUBSTANTIALLY, UNIFORMLY-DISTRIBUTED INSULATING BINDER-MATERIAL CONTAINING PARTICULATE PHOTOCONDUCTIVE ZINC OXIDE (II), THE NUMBER OF SAID SMALL SPOTS BEING BETWEEN 10**2 AND 300**2 PER SQUARE INCH, THE SAID INSULATING BINDER-MATERIALS OF SAID COATINGS BEING OF SUCH NATURE THAT SAID COATINGS HAVE A LOW DECAY RATE IN THE DARK WHEN SAID COATINGS HAVE RECEIVED IN ELECTROSTATIC CHARGE. 